1. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to base station transmit power control in a cellular radiotelephone system.
2. Description of the Related Art
A cellular radiotelephone system uses numerous base stations located throughout a geographical region. Each base station is typically equipped with three to six directional antennas to cover three to six different sectors of the cell. The antennas are typically located in the center of the cell.
When a mobile radiotelephone moves from one sector of a cell to another sector or from one cell to another cell, the radiotelephone must be handed off to that sector or cell. In code division multiple access (CDMA) cellular technology, this is typically a soft hand-off if the new sector/cell is within the same system as the sector/cell from which the mobile radiotelephone is moving.
A soft hand-off is analogous to a make-before-break switch. In other words, the mobile radiotelephone is communicating with both the old and the new base stations at the same time. After reliable communication is established with the new base station,.the old base station is no longer relied upon as the primary base station.
On the reverse link, the link from the radiotelephone to the base station, the fact that more than one base station is receiving the radiotelephone's signal results in a reduced frame error rate for a given transmit power. This is a form of diversity reception that is well known to improve frame error rate in digital communication systems. Communicating with more than one base station could also result in a lower required transmit power for a given frame error rate. Both effects are highly desirable.
Additionally, the more base stations receiving a radiotelephone's signal, the lower the required transmit power for the radiotelephone. On the reverse link, in other words, it is better for the radiotelephone to be communicating with the greatest number of base stations possible during soft hand-off.
However, on the forward link, increasing the number of base stations communicating with the radiotelephone during soft hand-off has an adverse effect that may work against the potential diversity gain. Forward link interference is increased due to the fact that each base station, communicating with the radiotelephone, is transmitting to the radiotelephone using the same power. The interference reduces forward link capacity. It is very important, therefore, to minimize the transmit power from the various involved base stations.
In particular, the amount of power transmitted from each base station has to be adjusted according to the link quality between a particular base station and the radiotelephone. As an example, in a two-way soft hand-off (i.e., a soft hand-off where the radiotelephone is communicating with two base stations at the same time), one of the base stations will likely have a better link than the other base station. Since the radiotelephone's Rake receiver, well known in the art, searches for and combines a certain number of the largest multi-path components of the total received signal, these components will probably come from the link with the smallest path loss. Therefore, the performance of the Rake receiver will be roughly the same as if the base station with the lower quality link did not transmit. Additional discussion of the Rake receiver is found in Proakis, John G., Digital Communications, Chapter 14 (3.sup.rd Ed. 1995).
Thus, if the second base station with the lower quality link is transmitting at the same power level as the first base station with the better link, the second base station may be causing unnecessary interference to the system. This shows that the base station with the better link should transmit the radiotelephone's signal at a higher power level than the other base station.
Unfortunately, a number of issues makes this problem more complicated than just increasing the power transmitted by the station with the better link. One issue is that the multi-path profiles (i.e., the number and relative strength of arriving signal echoes or copies) from the two base stations are not the same. Thus, even though one link may have a larger path loss, the combined received signal may still have strong multi-path components from both base stations.
Another issue is that one base station should not stop transmitting since, if the radiotelephone suddenly goes into a shadow of the good link, the call will be lost. Both stations, therefore, should be received with a power that is above a certain threshold to improve robustness.
Additionally, neither base station can transmit the radiotelephone's signal at a power level higher than is permitted by the excess capacity of each base station. This implies that each base station should transmit the radiotelephone's signal at a power level that is below a certain maximum threshold value that is different for each base station.
There is a resulting unforeseen need for an optimal power allocation process for radiotelephones during soft hand-off. This power allocation process should minimize the power transmitted by both base stations while not degrading the bit error rate below a certain threshold that degrades call quality.